首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1074篇
  免费   70篇
  国内免费   3篇
林业   135篇
农学   24篇
基础科学   7篇
  364篇
综合类   53篇
农作物   65篇
水产渔业   154篇
畜牧兽医   267篇
园艺   21篇
植物保护   57篇
  2024年   3篇
  2023年   17篇
  2022年   15篇
  2021年   33篇
  2020年   43篇
  2019年   53篇
  2018年   44篇
  2017年   47篇
  2016年   54篇
  2015年   33篇
  2014年   44篇
  2013年   62篇
  2012年   85篇
  2011年   102篇
  2010年   49篇
  2009年   46篇
  2008年   84篇
  2007年   71篇
  2006年   59篇
  2005年   42篇
  2004年   45篇
  2003年   31篇
  2002年   39篇
  2001年   3篇
  2000年   2篇
  1999年   3篇
  1998年   2篇
  1997年   4篇
  1996年   2篇
  1994年   2篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1990年   3篇
  1989年   2篇
  1987年   1篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1977年   1篇
  1975年   1篇
  1972年   1篇
  1971年   2篇
  1969年   1篇
  1948年   1篇
  1940年   1篇
  1930年   1篇
排序方式: 共有1147条查询结果,搜索用时 0 毫秒
71.
Wild raptors brought into an ex situ environment often have poor semen quality that is further compromised by urine contamination. Generally, it is believed that in birds, artificial insemination into the cloaca or caudal vagina of females requires large doses of high-quality spermatozoa to maximize fertility. In an effort to define and overcome some of the challenges associated with reproduction in wild raptors, the objectives of this study were to 1) evaluate the frequency, impact, and remediation of urine contamination in fresh ejaculates for the purpose of maintaining sperm motility and viability in vitro, and 2) develop a deep insemination method that allows low numbers of washed sperm to be placed directly into the magnum to increase the probability of producing fertilized eggs. The species evaluated include golden eagle (Aquila chrysoetos), imperial eagle (A. adalberti), Bonelli's eagle (Hiernaetus fasciatus), and peregrine falcon (Falco peregrinus). Semen samples were collected and pooled by species, and a minimum of 25 pooled ejaculates per species were evaluated for urine contamination, pH, sperm viability, and sperm motility; the samples were either unwashed or washed in neutral (pH 7.0) or alkaline (pH 8.0) modified Lake's diluent. Female golden eagles and peregrine falcons were inseminated via transjunctional, intramagnal insemination with washed spermatozoa from urine-contaminated samples. Urine contamination occurred in 36.8 +/- 12.8% (mean +/- SEM) golden eagle, 43.1 +/- 9.1% imperial eagle. 28.7 +/- 16.1% Bonelli's eagle, and 48.2 +/- 17.3% peregrine falcon ejaculates. The pH in urine-contaminated semen samples ranged from 6.48 +/- 0.3 to 6.86 +/- 0.2, and in noncontaminated samples it ranged from from 7.17 +/- 0.1 to 7.56 +/- 0.1. Sperm viability and motility were reduced (P < 0.05) in all species for unwashed vs. washed sperm after 30 min incubation at room temperature. Two peregrine falcon chicks and one golden eagle chick hatched after intramagnal insemination. This study demonstrates that urine contamination, a common and lethal acidifier in manually collected raptor ejaculates, can be circumvented by immediate, gentle seminal washing. Furthermore, these processed sperm, when deposited by transjunctional intramagnal insemination, can produce live young.  相似文献   
72.
A new sunflower mutant, CAS-12, was obtained, which has both high palmitic (≈30%) and high oleic acid contents, and also a substantial amount of palmitoleic acid (≈7%). The mutant was selected after X-ray irradiation of dry seeds of the inbred line BSD-2-423, which had normal palmitic (≈3%) and high oleic (≈88%) acid levels. The increase of palmitic and palmitoleic acids occurred at the expense of the oleic acid content, which decreased to around 55% in respect to the original line. Linoleic acid content is always under 5%. Palmitic and palmitoleic acid levels were similar to those of the high palmitic mutant CAS-5 obtained in a previous programme from a low oleic line isogenic to BSD-2-423 using a similar mutagenic treatment. In that previous programme we also selected three high stearic acid mutants using chemical mutagenic treatment on the same sunflower line (RDF-1-532). We attempted to obtain mutants in other lines but were unsuccessful. The isolation of similar mutants in isogenic parental lines illustrates the importance of the genetic background in the development of specific mutants with an altered seed oil fatty acid composition. The oil of this mutant will increase the range of potential uses of sunflower oil. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
73.
74.
Summary A method for comparing locations as selection sites based upon their abilities to predict yield and disease reaction over a target region is proposed. The probability of coincidence in selection for a site is defined as the probability for a line selected at the site to be selected at other sites within the region. The probability of divergence in rejection is defined as that associated with regional selection of a line given that is discarded by the site where selection is being conducted. The ideal selection site would maximize the probability of coincidence in selection and minimize the probability of divergence in rejection. The method is illustrated using a set of data from the rice yield nurseries of the International Rice Testing Program for Latin America planted under the rainfed conditions of Central América and México during the period 1978–1984. Five locations were compared for their predictive ability in selecting for the rainfed rice growing region, based on yield and disease reaction. Selection for yield was defined as performance superior to the best check in each location. Selection for disease reaction was based on an index derived from the Standard Evaluation System for Rice for diseases of regional importance. Locations varies 10–15 percent in their selection coincidence with the region for both yield and disease selection criteria applied independently.  相似文献   
75.
Winemaking waste solids (WS, resulting from red grapes after fermentation and distillation to recover spirits) were subjected to various processing schemes for isolating fractions with antioxidant activity. The liquors entrapped in WS as received were separated by pressing and freeze-dried to yield a fraction with antioxidant activity (measured as DPPH radical scavenging capacity) comparable to those of synthetic antioxidants. A second approach based on the direct processing of raw WS in sulfuric acid medium under fixed operational conditions and further extraction of hydrolysis liquors with ethyl acetate enabled the isolation of a fraction with higher antioxidant ability at an improved yield. The most favorable approach started with a washing stage leading to liquors (which were directly freeze-dried to yield 1.20 g of extract/100 g of oven-dry WS and presented an EC50 of 0.41 g of extract/L) and washed solids, which were dried and subjected to hydrolytic processing (i) with water as a reactive in an autocatalyzed reaction (autohydrolysis) or (ii) with sulfuric acid solutions to give an ethyl acetate-soluble fraction with improved antioxidant properties (EC50 in the range of 0.18-0.40 g/L). Samples from washing liquors and processing of washed solids in aqueous medium were subjected to chromatographic fractionation and analysis to give isolates with remarkable antioxidant activity (with EC50 as low as 0.07 g/L) and to identify their major components.  相似文献   
76.
The effect of micellar aggregates upon the stability of carbofuran in basic media has been studied. The effect of the presence of micelles upon the basic hydrolysis of carbofuran is a function of the nature of the surfactant monomer. Important catalysis of basic hydrolysis of carbofuran in the presence of colloid aggregates with positive surface charge has been reported. On the other hand, the presence of anionic and nonionic surfactants implies a large inhibition of the basic hydrolysis of carbofuran. Both catalysis and inhibition are due to the association of carbofuran with the micellar core. The kinetic constants for the basic hydrolysis of carbofuran in these microheterogeneous media have been obtained on the basis of a micellar pseudophase model. No significant changes in the intrinsic reactivity of HO- against carbofuran have been observed.  相似文献   
77.
Winemaking with selected yeasts requires simple and cheap techniques to monitor the yeast population dynamics. We obtained new sulfometuron (smr) resistant mutants, easy to detect by replica-plate assay, from selected wine yeasts. The mutations were dominant and were located at the ilv2 locus that encodes for acetolactate synthase enzyme. The mutants were genetically stable and maintained the killer phenotype of the parent yeast strain. They were genetically improved by elimination of recessive growth-retarding alleles followed by spore clone selection according to the must fermentation kinetics and the organoleptic quality of the wine. Some mutants were tested in industrial winemaking and were easily monitored during must fermentation using a simple plate assay. They accounted for more than 95% of the total yeasts in the must, and the resulting wine had as good a quality as those made with standard commercial wine yeasts.  相似文献   
78.
Changes in 36 volatile compounds of must from ripe grapes dried by direct exposure to sun and must from ripe grapes were studied. Compounds not dependent on sampling site in both musts were selected, and their concentration/Brix degree ratio values, were subjected to variance analysis. Only butan-1-ol and isoamyl alcohols showed no differences, while (E)-hex-3-en-1-ol, (Z)-hex-3-en-1-ol, (E)-hex-2-en-1-ol, (E)-hex-2-enal, hexanoic acid, isobutanol, benzyl alcohol, 2-phenylethanol, gamma-butyrolactone, gamma-hexalactone, and 5-methylfurfural, showed significant differences between the two must types, which may be ascribed to the drying process. An approach to describe must odor has been carried out by grouping volatile compounds in aromatic series, increasing their values in the fruity, solvent, sweet, and roasted series and diminishing the herbaceous as a consequence of the drying process.  相似文献   
79.
The effect of the addition of Lactococcus lactis subsp. lactis INIA 415, a strain harboring the structural genes of nisin Z and lacticin 481, on the formation of volatile compounds in Hispánico cheese manufactured with a mesophilic starter or with the mesophilic starter and a thermophilic starter was investigated. Addition of bacteriocin-producing L. lactis subsp. lactis INIA 415 to milk enhanced the formation of 2-methyl-propanal, 2-methylbutanal, 3-methylbutanal, 2-methyl-1-propanol, 3-methyl-1-butanol, 1-octanol, 2-butanone, and 2,3-butanedione. On the other hand, addition of thermophilic starter enhanced the formation of acetaldehyde, ethanol, 3-methyl-2-buten-1-ol, ethyl butanoate, ethyl hexanoate, 2-butanone, and 2,3-butanedione in Hispánico cheese. Stepwise discriminant analysis using the relative abundances of volatile compounds classified cheeses by type of starter, with function 1 related to thermophilic starter and function 2 to bacteriocin producer.  相似文献   
80.
Abstract

Foliar analysis is an effective method to diagnose the nutritional status of plants. However, the mineral concentration in foliar tissue has traditionally been evaluated by assessing the activity of each element, without considering the interactions between them. To address this, dual interactions were calculated using the Diagnosis and Recommendation Integrated System (DRIS) to identify which crop nutrients are most influential in nutrient imbalances and which are the most limiting nutrients for the nutritional status of banana crops in Ecuador. To achieve this, a regional survey of the nutritional status and its productivity levels was conducted for 188 different sites during the crop season in 2017–2018, involving banana cultivars ‘Vallery’ and ‘Williams’, from the Cavendish subgroup. The DRIS calculation method was combined with Beaufils and Jones functions. From the initial 188 foliar samples, 83 samples (representing 44% of the population) were considered to represent the high-yield reference population, with yields of 38–60 t ha?1. The DRIS method defined the mean nutritional balance index, which was not found to be statistically correlated (p?>?0.05) with productivity, revealing that there was no significant association with the nutritional status of the plants. Specific DRIS norms were obtained and indicated that deficiencies in K, N, Ca, and Fe, and excesses in Mn, B, Cl, Zn, S, Cu, and Mg were the most limiting nutrients for banana cultivars in the south of Ecuador.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号